Several criteria must be met in a well-formulated emulsion. Two of the most readily apparent requirements are that the emulsion possesses adequate physical and chemical stability.
First of all, chemical stability of the various components of the emulsion should receive a particular attention in perfumery, since the main materials used in perfuming compositions, namely perfuming ingredients, are well-known to be particularly difficult to stabilise in formulations such as emulsions (Cosmetics and Toileteries® Magazine, Vol. 109, pages 71–75, 1994). The stabilisation of formulations comprising high fragrance levels such as those concerned by the present invention constitutes therefore a specific and challenging problem, notably different from that of the stabilisation of a cosmetic emulsion, which comprises other kinds of components, the nature of which is generally dependent on the type of the final cosmetic product (typical active ingredients include natural or synthetic oils, hydrocarbons, halogenated carbons, esters of mineral acids, silicones). Furthermore, such cosmetic compositions contain very low amounts of perfuming ingredients.
On the other hand, an essential requirement sought in the formulation of an emulsion of any kind is the physical stability, without which the emulsion will soon revert back to separate bulk phases. One typical phenomenon associated with physical instability is constituted by the upward and downward movement of dispersed droplets relative to the continuous phase, termed creaming or sedimentation respectively. The factors that influence the rate of sedimentation or creaming are the diameter of the suspended droplets, the viscosity of the suspending medium and the difference in densities between the dispersed phase and the dispersion medium.
The prior art discloses a first means to avoid this destabilisation phenomenon, consisting in reducing the particle size of the dispersed system, which contributes greatly toward overcoming or minimising creaming.
Therefore, perfuming compositions in the form of microemulsions are known from the prior art. Microemulsions are liquid dispersions of water and oil that are made homogeneous, transparent and stable by the addition of relatively large amounts of a surfactant and a co-surfactant. While emulsions are unstable, microemulsions are stable and, therefore, they are formed spontaneously when oil, water, surfactants and co-surfactants are mixed together. Conversely, emulsions require input of considerable mechanical energy for their preparation. Furthermore, microemulsion droplets have a narrow droplet size distribution with a mean diameter range of approximately 5–50 nm. Consequently this type of formulation scatters little light and is therefore transparent or at least translucent, while emulsions have very broad droplet size distributions and, as the emulsion droplets are comparable in size or larger than the wavelength of visible light, they scatter it more or less strongly depending on the difference in refractive index between oil and water. Thus most emulsions are opaque. It appears therefore that emulsions and microemulsions constitute two distinct dispersed systems, of different nature for the reasons here-above mentioned.
Now, for the purpose of the present invention, it is not desirable to incorporate an important amount of surfactant relative to the amount of perfuming ingredients, as this limits considerably the amount of fragrance that can be incorporated in the mixture. Therefore, a microemulsion does not constitute a suitable dispersed system for the present invention, which has to be in the form of an emulsion.
Another known transparent dispersed system of this type is the nanoemulsion characterised by an average size of the oily phase droplets below ca. 30–75 nm. The droplets are small enough to make the emulsion translucent or partially transparent. Although these nanoemulsions present the advantage of needing lesser amount of surfactants than the microemulsions, they present nevertheless the disadvantage that their process of preparation is often difficult and delicate, and thus costly.
The other manner disclosed in the prior art to stabilise a conventional emulsion is the addition of a polymer to the dispersed phase. However, this solution presents the drawback of thickening said phase, thus increasing the viscosity of the emulsion such that it is no longer easily sprayable.
WO 00/33804, the entire content of which is hereby incorporated herein by reference and which describes a perfuming composition in the form of a transparent emulsion, already matches most of the criteria required for such an emulsion (transparency, fluidity). However, the emulsion described in that publication has now been improved in terms of physical stability, thanks to a novel solution to the physical stability problem described herein.
In fact, the invention now provides a transparent, fluid, water-in-oil or oil-in-water emulsion characterised by the fact that the densities of the respective continuous and dispersed phases have almost matching values. This solution to the well-known problem of physical instability, and more particularly of creaming and sedimentation, specifically adapted to a dispersed system of the emulsion type, has been achieved thanks to the presence of a specific additive in the oily phase of the emulsion, namely a volatile fluorinated oil having a density higher than 1.
This class of compounds has already been used in some kinds of emulsions. In particular, EP 1064929 discloses the use of such a volatile fluorinated oil in a cosmetic composition for attenuating skin deficiencies and for giving a matt skin appearance. The cosmetic composition there-disclosed is not only totally distinct, from a formulation point of view, from a perfuming composition, but also the fluorinated oil is used in this prior art for a technical effect consisting in modifying the skin appearance, thus an aim distinct from that achieved by the present invention, i.e. the stabilisation of an emulsion.
EP 1064989 also discloses a cosmetic emulsion comprising a volatile fluorinated oil. Once again, the latter is described as being useful for its topical properties in such cosmetic applications, providing compositions of easy application, as well as fresh and soft products. Moreover, this document specifies that the fluorinated oil is very difficult to stabilise and may provoke an instability phenomenon such as foaming, demixing and changes in viscosity, and that it has thus to be used in combination with a specific surfactant, able to stabilise the emulsion. According to the teaching of this document, a volatile fluorinated oil is thus very difficult to stabilise.
The present invention now seeks to resolve these deficiencies.